Reinforced plastics are combinations of fibers and polymeric binders or matrices that form composite materials. This combination can achieve a balance of material properties that are superior to the properties of either single material. In the past, reinforced plastics have been utilized largely by aircraft, marine, automobile, and chemical manufacturers.
The combination of strong fibers and synthetic polymers to form reinforced plastics and laminates derives from several basic considerations of material science: the inherent strength of fine fibers, the wetting requirement for adhesion between the fibers and a matrix, and the ease of the liquid-solid phase change of synthetic polymers. In a general sense, the polymeric matrix serves the purpose of a supporting medium surrounding each fiber and separating it from its neighbors, and stabilizing it against bending and buckling. These functions are best fulfilled when there is good adhesion between the fibers and the matrix. Adhesion can be fostered by utilizing a relatively low-viscosity liquid polymeric precursor to impregnate a reinforcing fiber material, followed by polymerization of the matrix. Adhesion can also be enhanced by plasma surface treatment of the fibers.
Two broad categories of polymeric materials have been typically utilized to prepare reinforced resins in the prior art. These two types are the thermoplastic polymers, which generally melt in the range of 150.degree. C. to 250.degree. C. and readily solidify upon cooling, and the thermosetting polymers which pass through a liquid phase just once during their life, while they are being polymerized and cross-linked into heat-infusible forms.
The two predominant types of fibers that have been used to reinforce plastics, considering all uses of composites, have been glass and cellulose fibers. Fibrous glass comprises well over 90% of the fibers used in reinforced plastics because it is inexpensive to produce and possesses high-strength, high-stiffness, low specific gravity, chemical resistance, and good insulating characteristics. In reinforced plastics, glass has been used in various forms. Advantageously, it has been chopped into short lengths (6-76 mm) and gathered into a felt or matte, resulting in a form that is easy to handle and low in cost. Previously, it has been observed that the best properties in the final composite have been achieved with nonwoven fabrics in which all the fibers are straight, continuous and aligned parallel in a single direction.
In addition to glass and cellulose fibers, other types of fibers have also been used to reinforce plastic materials. The stiffest fibers known are composed of graphite, which theoretically can be almost five times more rigid than steel. However, despite much work over many years by many technical organizations, the cost of graphite fibers remains high. As a result, their use in composites is limited to applications that place a premium on weight savings: aircraft, missiles, sports equipment, etc.
In 1971, aromatic polyamide fibers became widely commercially available and are presently being used extensively in automotive tires and numerous aerospace structures. The aromatic polyamides are designated as aramids by the Federal Trade Commission, and that is the term used herein to refer to them. One specific aramid that has been widely used in many applications is referred to as Kevlar.TM.. Discovered in 1965, Kevlar.TM. is produced and marketed by DuPont.
In the stiffness range between glass and steel, aramids are lighter than glass, comparatively strong, and much tougher and absorb considerable energy before breaking, even under impact conditions. The fibers are highly crystalline and directional in character. Kevlar.TM. fibers are known to have excellent resistance to flame and heat, organic solvents, fuels and lubricants, and they can be woven into fabric. Because of their strength and other properties, aramid fibers have been used in sports equipment, and in protective systems where ballistic stopping exploits their superior impact resistance.
More recently, fibers of ultra-high strength polyethylene have been produced. Such fibers are available from Allied Signal, Inc., Fibers Division, Petersburg, Va., under the trademark Spectra.TM., and Dutch State Mining Corporation. The fibers are made of extended chain polyethylene and have a low specific gravity of about 0.97, which is less than the specific gravity of fiberglass or aramid fibers.
Although composites have been used in the past in a number of settings, their use has not been fully exploited in all fields. The present invention involves the use of a lightweight woven fabric, such as an aramid (e.g. Kevlar.TM.) or a polyethylene (e.g. Spectra.TM.) to reinforce resinous portions of dental structures, such as dental prostheses and other restorative appliances. Previously, the possibility of using a lightweight woven fabric in synthetic resinous portions of dental structures has not been reported.
The following publications exemplify prior uses of aramids in the context of dental applications:
European Patent Application No. 0,221,223 discloses a magnetic retaining device for dental prostheses, comprising magnets intended to be implanted in the upper or lower jawbone within casings of a biocompatible material, and corresponding elements incorporated in the prosthesis which are magnetically attractable by the implanted magnets. According to this European patent application, the prostheses can be rendered lighter by forming the prostheses from a hollow body of resin which carries the teeth, the cavity of which is filled with a mass of composite material of resin and reinforcing fibers, normally glass fibers or Kevlar.TM.. It is likely that in the context of the prior art, those working in the dental field would likely have selected relatively short fibers of glass or Kevlar.TM. for the reinforcement purposes described in this European patent application.
On the other hand, as discussed in greater detail hereinbelow, the present invention relates to the use of a woven fabric of an aramid (such as Kevlar.TM.) or of a polyethylene (such as Spectra.TM.) to reinforce resinous portions of dental prostheses and dental appliances, and to particular methods tailored to producing such reinforced resin-containing dental structures.
Kawahara et al., U.S. Pat. No. 4,731,020 discloses a removable denture retaining structure which is mounted on an elastic member that is located between the denture body and a support base. The patent further discloses that the elastic support member may be reinforced with a wide variety of organic fibers, ceramic fibers, glass fibers, etc. However, this patent does not relate to reinforcement of the dental prosthesis or appliance itself, as with the present invention. Moreover, the resins to be reinforced in accordance with the present invention are preferably nonelastomeric, in direct contrast to the elastomeric mounting member of Kawahara et al.
Kawahara et al., U.S. Pat. No. 4,738,622, contains essentially the same disclosure as Kawahara et al., discussed above.
Goldberg et al., U.S. Pat. No. 4,717,341, is directed to an orthodontic appliance system, the components of which are formed from fiber reinforced composite material comprising a polymeric matrix, and at least 5% of a reinforcing fiber embedded in the matrix. The patent states that although a variety of fibers may be employed, the most commonly utilized fibers are glass, carbon and/or graphite and aramid fibers (referred to in the patent as polyaramid fibers). In contrast to the present invention, the thrust of the Goldberg et al. patent, and all of the examples therein, relate to the use of unwoven fibers (especially glass, but also "aramid") to reinforce appliances, rather than woven fabrics.
Also, the Goldberg patent relates to reinforcement of the force-imparting portions of orthodontic appliances: primarily wires, but also including arches, segments, hooks, tie-backs, ligature wires and springs, pins, brackets, tubes, active lingual appliances, etc. These appliances are designed and configured to exert active force on a natural oral structure such as a tooth. The reinforcement of the dental prostheses and appliances of the present invention involves non-force-imparting portions thereof. It should be noted that woven fabric reinforcements are incompatible with wires and springs.
Ferraro et al., U.S. Pat. No. 3,957,067, and Wolak, U.S. Pat. No. 4,836,226, are each directed to dental floss or dental floss-like articles in which one of the materials that could be used to make the article is Kevlar.TM.. Neither of these patents is directed to reinforced resins, and they are cited herein only because they disclose another use of Kevlar.TM. in a dental setting.
In spite of the above-described prior art, a number of possible types and applications of composite resins in the dental field have not been described or suggested.
It is therefore an object of the present invention to provide reinforced dental structures which include at least a portion made of a resinous material.
It is yet another object of the present invention to provide a method for reinforcing resin-containing dental structures.
Another object of the present invention is to provide a reinforcing material having superior bonding and strength-imparting properties.